There are many chemical agents that need to be detected and monitored that are not conveniently handled by conventional chemical detection apparatti such as chromatographs or spectrometers. Especially inconvenient are chemicals that are solids at room temperature since they must often be dissolved in a solvent before an analysis can be performed. Analytical chemistry techniques are not helpful with these agents because they require skill, often lack great sensitivity or selectivity, and are not well suited for incorporation into portable, direct-reading instruments.
A typical example of such agents is the illicit drug cocaine hydrochloride. This material has an extremely low vapor pressure which makes it nearly impossible to detect as a vapor with popular vapor sniffers such as ion mobility spectrometers, gas chromatographs, electrochemical sensors, etc. Thus, when searching for cocaine hydrochloride, the drug will only be found as minute particles, or possibly as an adsorbate on ambient aerosol particles.
One possibility that shows great potential for monitoring ambient air for the presence of chemical vapors of interest is microsensor technology. See H. Wohltjen et al., "Chemical Vapor SAW Microsensor Array for Application in Drug Interdiction: Instrument Design and Development", Proc of Int'l Symp. on Substance Identification Technologies, Innsbruck, Australia (Oct. 4, 1993). The use of chemical microsensors to monitor gas has been the subject of U.S. Pat. No. 4,759,210, granted Jul. 26, 1988, to Microsensor Systems, Inc., the subject matter of which is incorporated herein by reference.
Chemical microsensors are generally defined as solid state, micro-fabricated electronic structures that respond to their chemical environment. They include a variety of devices, such as surface acoustic wave (SAW) devices, organic and inorganic semiconductors, ChemFETs, microelectrode arrays for electrochemical measurements, and other electronic structures. Even though each type of microsensor may respond in a different way to a chemical environment, they share many desirable features. For example, they are all very small, sensitive, rugged, relatively inexpensive, low maintenance, and provide an electrical signal that can be readily integrated into a measurement system.
One class of chemical microsensor, the surface acoustic wave ("SAW") device, has received increasing attention in the research and development community, and is being incorporated into many prototype chemical monitors for introduction to the field. SAW devices were first proposed as sensors for chemical vapors in 1979. H. Wohltjen and R. E. Dessy, "Surface Acoustic Wave Probe for Chemical Analysis, I. Introduction and Instrument Design", Anal. Chem., 51 (9): 1458-1464 (1979). Since then many studies have been undertaken to improve their sensitivity by increasing operating frequency or by improving device configuration, such as operating the SAW devices in the resonant mode rather than as delay lines. The selectivity of SAW devices for specific chemicals has also been improved over the years through the development of better surface coatings and the use of SAW sensors in multiple sensor arrays (with each SAW device having a different chemically sensitive coating). This array of sensors can be coupled to a pattern recognition processor to enhance the operational selectivity of the sensor system. These pattern recognition processor systems employ a pattern recognition algorithm to analyze data fed to the processor from the array of sensors when those sensors come in contact with chemical species which it is desired to detect. See H. Wohltjen, "Mechanism of Operation and Design Considerations for Surface Acoustic Wave Vapor Sensors," Sensors and Actuators, 5(4):307-325 (1984).
SAW microsensors and SAW microsensor arrays have most recently been incorporated into a number of novel applications, i.e. chemical warfare (CW) agent detectors. One of these CW agent detectors was a "Smart Sensor" SAW array system built for the Air Force. It utilized four SAW devices, each with a different chemically sensitive coating. The combination of the four SAW sensors with pattern recognition proved to be very sensitive and selective for various chemical warfare agents with effective discrimination against anticipated interfering vapors. The sensitivity of detecting and identifying some agents were as low as 0.01 mg/m.sup.3 in a two minute analysis. Even lower detection limits were possible using longer sample concentration times. Above their threshold detection limits, each CW agent could be identified 100% of the time, even when present in mixtures with other vapors at concentrations that were 50 times higher than the agent.
However, while the aforementioned technology is quite beneficial, it does not work effectively with non-volatile substances. This has frustrated that technology's application to the investigation of samples of interest for such non-volatile substances, and particularly for illicit substances of that sort such as cocaine hydrochloride and heroin. Thus, the art has not taken appropriate advantage of this powerful analytical tool.
The development of a convenient and accurate method and apparatus for investigating samples of interest to determine whether they contain non-volatile analytes would be a significant step forward in the art.